Decreasing regional variations in exposure levels seen by an X-ray detector can yield improvements X-ray image quality; since a detector can be limited to a predetermined dynamic range, minimizing the difference between the most exposed and least exposed regions of the detector can improve its sensitivity to slight differences in exposure, e.g., the contrast, or avoid having saturated or completely dark regions in an image. It can be particularly difficult to control dynamic range in computed tomography (CT) applications due to the plurality of different projection images seen by the detector in a single scan. In CT, projection images can be acquired through more than 180 degrees around a patient, and the profile of a patient can be much narrower in some projections than others, e.g., narrower viewed from shoulder to shoulder than front to back. In narrow projections the detector may be highly exposed or over exposed, e.g., on the edges. Conversely, wide projections may receive a higher dose than necessary to penetrate the patient in that direction, as a patient may be thinner in a front to back orientation than in a side to side projection. Narrowing dynamic range in CT may both improve image quality and decrease patient X-ray dose.
One strategy that has been implemented to address dynamic range problems in CT is placement of a bowtie filter between an X-ray source and the patient during imaging. A bowtie filter has been a physical filter with a shape that is relatively thick near the edges and thin at its center, with a linear, parabolic, circular, or any other type of gradient between these maxima and minima. Use of a bowtie filter can also provide benefits for patient X-ray dose reduction, which has been a goal of the medical imaging community over the past decades. Without use of the filter, achieving enough photons for adequate noise performance at the center of the detector, where in some projections the patient may be the thickest, may result in the outer regions of the detector receiving an unnecessarily large number of photons. This large number of photons may be detrimental to image quality any may also contribute to excess patient dose.
While bowtie filters can have many advantages, their utility is also limited by a lack of adaptability. For example, to accommodate a range of patient sizes, a set of bowtie filters of variety of sizes may be provided and the closest match can be selected for use with each patient. However, the closest match from a premade set may not be an exact or ideal match for each patient. The thickness of a patient profile also can vary with projection angle, such that a single bowtie filter cannot achieve optimal results at all projection angles throughout a CT scan of a patient. A limited number of relatively complex bowtie filters have been proposed for adaptive bowtie filtration. Examples include a piecewise-linear dynamic bowtie filter proposed by Hsieh et al. (Scott S. Hsieh; Norbert J. Pelc. “The feasibility of a piecewise-linear dynamic bowtie filter.” Med. Phys. 40, 031910 (2013)), which utilizes a plurality of precisely controlled wedge-like pistons to implement piecewise triangular function and a dynamic bowtie filter comprising a pair of sliding wedges proposed by Szczykutowicz et al (Timothy P. Szczykutowicz; Charles Mistretta. “Intensity Modulated CT implemented with a dynamic bowtie filter.” Proc. SPIE 8668, Medical Imaging 2013: Physics of Medical Imaging, 866818 (Mar. 19, 2013)).
Embodiments of the present invention can provide adaptive bowtie filtration with relatively simple and fast implementation methods and enhanced flexibility relative to existing systems.